1. Field of the Invention
This invention relates in general to liquid crystal display devices, and more particularly, to bistable twisted-nematic liquid crystal devices.
2. Discussion of the Background
Liquid crystals, which include ordered molecules or groups of molecules in a liquid state, are found to be considerably useful for fabricating devices for switching, modulating and otherwise altering characteristics of light beams. Differences in transmittance and in a polarizing effect of such liquid crystals both have been now utilized for, for example, liquid crystal displays for audio equipment, instrument panels and office automation equipment.
However, it would be more practical for a number of new applications to have a liquid crystal material which has two stable states, and which can easily transform from one stable state to the other, rapidly and with a minimum expenditure of energy.
To implement a high speed drive for liquid crystal devices, a variety of liquid crystal displays using bistable twisted-nematic liquid crystals have been disclosed as exemplified in Japanese Published Patent Application No. 1-51818 and Japanese Laid-Open Patent Application No. 6-230751.
Bistable characteristics are shown for twisted-nematic liquid crystals in these disclosures, in which at least two pulse voltages are applied to produce an electric field across a liquid crystal cell. A first pulse is used to initiate a Freedricksz transition of the liquid crystal and a second pulse is used to subsequently relax the liquid crystal into one of two metastable states, thereby modulating optical transmittance or reflectivity to be utilized for display devices.
Although principles for switching behavior of possible displays are presented in JP 1 -51818, no description is made on driving the displays. Also, JP 6-230751 proposes basics of driving simple matrix type displays. However, no description is made for a gray scale technique of display pixels, which is deemed essential to high quality liquid crystal displays.
In addition, Japanese Laid-Open Patent Application No. 8-313878 proposes a gray level modulation technique in which gray levels of display pixels may be obtained by applying pulse voltages to scan lines and by changing a ratio of two metastable states during a scan period. However, since the pulse voltages are applied to an entire scan line by the above technique, this results in the same gray level in display pixels on that scan line. Although a different gray level in an individual pixel on a single scan line may be feasible by (1) superposing on- and off-states in pixels and (2) modulating applied potentials over a plurality of display picture frames, a maximum transmittance (or reflectivity) intrinsic to a liquid display panel can be achieved only to a certain extent.